1. Field of the Invention
The present invention relates to a wireless mobile communication system. More particularly, the present invention relates to a method for signal transmission/reception based on a Hybrid Automatic Repeat reQuest (HARQ) scheme in a wireless mobile communication system.
2. Description of the Related Art
Currently, development of mobile communication systems is oriented toward provision of various services, including broadcast, multimedia images, and multimedia messages, to users. More particularly, next generation mobile communication systems are being developed in order to provide a data service of at least 100 Mbps for high speed mobile users and to provide a data service of at least 1 Gbps for low speed mobile users.
Reliable high speed data transmission/reception between a base station and a mobile station in a wireless mobile communication system requires a small control overhead and a short latency. As a method for reducing the control overhead and supporting the short latency, a synchronous HARQ can be taken into consideration. The synchronous HARQ refers to an HARQ operation having a predefined corresponding relation between an initial signal transmission and a signal retransmission thereafter based on the HARQ scheme. Such a corresponding relation is called an HARQ transmission timing structure or an HARQ interlace structure. The HARQ interlace structure includes a relation between a time slot, in which a MAP indicating transmission is provided, and a time slot, in which a signal corresponding to the MAP is transmitted, a relation between a time slot, in which the signal is transmitted, and a time slot, in which a corresponding feedback is transmitted, and a relation between a time slot, in which the feedback is transmitted, and a time slot, in which a signal corresponding to the feedback is transmitted (i.e. retransmission).
In a wireless mobile communication system using the HARQ scheme, when a transmitter transmits a signal, a receiver transmits an ACKnowledgement (ACK) or a Non-ACKnowledgement (NACK), which indicates if the receiver has successfully received the signal, to the transmitter. Based on the ACK or NACK, the transmitter transmits a new signal or retransmits a previously-transmitted signal according to the HARQ scheme. As used herein, the HARQ scheme implies a Chase Combining (CC) scheme or an Incremental Redundancy (IR) scheme.
As described above, the HARQ scheme causes a time delay between initial transmission of a signal by a transmitter and transmission of a new signal or retransmission of the original signal by the transmitter according to reception of the ACK or NACK.
FIG. 1 illustrates a retransmission delay according to a conventional HARQ.
Referring to FIG. 1, the transmitter Tx transmits a data burst in the time slot of number 4, the receiver Rx transmits an ACK or a NACK to the transmitter at the time slot of number 8 according to existence or absence of a detected error in the received data burst. The example illustrated in FIG. 1 is based on an assumption that the receiver transmits a NACK. Upon receiving the NACK, the transmitter retransmits the data burst, which has been transmitted at the time slot of number 4 and at the time slot of number 12. In FIG. 1, the HARQ ReTransmission Delay (RTD) refers to the time between previous transmission and current transmission and includes eight time slots from the time slot of number 5 to the time slot of number 12. In the meantime, the Reception delay (Rx delay) refers to a time interval between reception of the data burst by the receiver and transmission of the NACK by the receiver after decoding of the data burst. The Rx delay includes three time slots from the time slot of number 5 to the time slot of number 7 in FIG. 1. The transmission delay (Tx delay) refers to a time interval between reception of the NACK from the receiver by the transmitter and transmission or retransmission of a data burst to the receiver by the transmitter. The Tx delay includes three time slots from the time slot of number 9 to the time slot of number 11 in FIG. 1. As described above, the HARQ retransmission time includes a reception delay, an HARQ feedback, a transmission delay, and a Transmission Time Interval (TTI) for transmission of a data burst.
In a communication system using the HARQ scheme, if transmission or retransmission delay time of a signal is maintained constant, the base station can use a persistent allocation scheme. According to the persistent allocation scheme, resources once allocated to a mobile station are continuously used during a predefined interval, it is thus unnecessary to transmit a control message indicating resource allocation whenever a signal is transmitted or retransmitted. However, the transmission or retransmission delay time of a signal may change according to the ratio between the number of DownLink (DL) time slots and the number of UpLink (UL) time slots. The time slot refers to a two-dimensional resource allocation unit determined by the time interval and the frequency band.
Table 1 and 2 below show HARQ signal transmission/reception according to the Mobile Broadband Frequency Division Duplex (MBFDD) scheme and the Mobile Broadband Time Division Duplex (MBTDD) scheme defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.20 standard. Table 2 has a temporal configuration ratio of M:N between the downlink interval and the uplink interval.
TABLE 1MAPData burstACKRetransmissionDLddd + 3d + 6ULuu + 2u + 6u + 8
TABLE 2MAPData burstACKRetransmissionDLN > 1ddd + 1d + 2N = 1ddd + 1d + 3ULM > 1uuu + 2u + 2M = 1u − 1uu + 2u + 3
In Tables 1 and 2, d corresponds to a downlink interval index, and u corresponds to an uplink interval index. Further, in the TDD communication system, M denotes the number of time slots occupied by the downlink data burst and N denotes the number of time slots occupied by the uplink data burst. Also, each interval occupies one time slot in the FDD communication system, while a downlink interval includes M time slots and an uplink interval includes N time slots in the TDD communication system.
As noted from Table 2, since the number of time slots in each interval changes according to the ratio of DL:UL (=M:N), the HARQ transmission or retransmission delay time is not constant. For example, for a 2:1 downlink data burst transmission, the transmitter indicates transmission of a data burst in the MAP within an interval of number 0, and transmits the data burst in the same time slot. Further, the transmitter receives a feedback signal through an uplink number 1 sub-frame, and retransmits the data burst through a downlink number 3 interval. Then, the downlink number 0 interval, the uplink number 1 interval, and the downlink number 3 interval configure one HARQ interlace structure.
FIGS. 2A and 2B illustrate a downlink HARQ interlace structure according to a conventional MBTDD scheme.
Specifically, FIG. 2A illustrates a structure when the ratio between the downlink and the uplink is 2:1, and FIG. 2B illustrates a structure when the ratio between the downlink and the uplink is 1:1. In FIGS. 2A and 2B, a downlink interval may include M time slots (TTIs) while an uplink interval may include N time slots. For example, in FIG. 2A, the number 0 time slot and the number 1 time slot correspond to the number 0 downlink interval, the number 2 time slot correspond to the number 0 uplink interval, and the number 0 downlink interval and the number 0 uplink interval correspond to the number 0 frame. Similarly, in FIG. 2B, the number 0 time slot correspond to the number 0 downlink interval, the number 1 time slot correspond to the number 0 uplink interval, and the number 0 downlink interval and the number 0 uplink interval correspond to the number 0 frame.
In the case of FIG. 2A, a Base Station (BS) transmits a data burst to a Mobile Station (MS) within the number 0 downlink interval. The number 0 downlink interval includes the number 0 time slot and the number 1 time slot. The MS receives the data burst from the BS, and demodulates and decodes the data burst. Due to a delay according to the demodulation and decoding, the MS may not feedback information on existence or absence of a detected error back in the number 0 uplink sub-frame, and instead transmits the feedback information on existence or absence of a detected error to the BS in the number 1 uplink sub-frame (specifically, the number 5 time slot).
Due to a delay for processing of the received feedback information on existence or absence of a detected error from the MS, that is, due to a delay for detection of an ACK or NACK, the BS may not transmit (or retransmit) a data burst in the number 2 downlink interval, and instead transmits (or retransmits) a data burst in the number 3 downlink interval.
It is noted from FIG. 2A that there are nine time slots before the data burst transmitted in the number 0 downlink interval is retransmitted in the number 3 downlink interval.
However, it is noted from FIG. 2B that there are six time slots before the data burst transmitted in the number 1 downlink interval is retransmitted in the number 3 downlink interval. This implies that the retransmission period may change according to the ratio between time slots of the downlink and the uplink.